This application claims Paris Convention priority of DE 101 08 065 filed Feb. 17, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a bone screw, in particular a pedicle screw, comprising a forked head with a groove for receiving a correction rod, a threaded screw shaft and a nut for the free ends of the forked head.
EP 0 699 055 B1 discloses an osteosynthesis device comprising a bone screw including a forked head into which a correction rod can be inserted. The correction rod is held by a head nut which is screwed onto a thread provided on the legs of the forked head. There are situations when screwing of the head nut can be difficult or e.g. present problems to a surgeon who cannot directly see the screw.
It is therefore the underlying purpose of the invention to further develop a bone screw of the above-mentioned type such that the nut and the forked head can be easily connected even in difficult situations.
This object is achieved with a bone screw of the above-mentioned type in accordance with the invention in that the free ends of the forked head and the nut form a bayonet joint.
In accordance with the inventive bone screw, the nut is disposed onto the free ends of the two legs of the forked head like a bayonet joint and slightly turned. This bayonet joint has the substantial advantage that the nut must no longer be mounted onto a thread but merely placed on the end of the forked head. This is easy to do even in difficult situations with insufficient sighting.
In a preferred further development, the free ends comprise radially outwardly extending projections. These projections extend through the peripheral length of the free ends of the forked head. This permits transmission of relatively large forces and the surface pressure is relatively small.
In accordance with the invention, the inner circumference of the nut comprises two radially inwardly projecting noses which each extend through 200 to 1200, preferably 850 of the inner periphery. The projections of the free ends of the forked head engage behind these noses to thereby form the bayonet joint.
In a further development, at least one free end of the forked head is provided with an axial extension. This axial extension defines the maximum turning of the nut to close the bayonet joint. Towards this end, the inner surface of the nut has a receiving groove for the extension extending through a portion of the inner circumference. When the nut is disposed onto the ends of the forked head, the extension engages into the receiving groove and the nut can be turned until the extension has reached the end of the receiving groove.
In another embodiment, the inner side of the nut is provided with a stop for the extension. In this embodiment, the stop engages the path of the extension such that the nut can be turned until the extension abuts the stop.
In a further development, the separation between the groove bottom of the forked head and the side of the nut facing the groove is larger than the inside diameter of the forked legs of the forked head, when the nut is disposed onto the forked head. This means that, when the correction rod has been inserted into the forked head and the nut is placed onto the free ends of the forked head, the nut is not supported on the correction rod. It can therefore be easily turned to close the bayonet joint.
In a further development, the nut is provided with a central threaded bore for a screw, in particular, a set screw. The correction rod is fixed in the forked head by this set screw which is screwed into the nut after closing the bayonet joint to press onto the correction rod. The screwed-in end of the set screw preferably comprises a tip or cupped gripping point which penetrates the surface of the correction rod. The set screw maintains the correction rod at the bottom of the forked head and secures the nut on the forked head to keep the bayonet joint in the closed position.
The screwing direction of the set screw preferably corresponds to the turning direction of the bayonet joint such that the nut must not be held when screwing in the set screw. Release of the nut is thereby relatively easy since the screwing direction of the set screw corresponds to the turning direction for opening the bayonet joint.
The outer circumference of the nut is provided with a tool engagement surface for turning the nut, in particular a hexagon, which permits easy movement of the nut in both turning directions to open and close the bayonet joint.
Further advantages, features and specifics of the invention can be extracted from the following detailed description of a particularly preferred embodiment with reference to the drawing. The features in the drawing, in the claims, and the description may be essential to the invention either individually or collectively in any arbitrary combination.